Asymmetric cell division: from A to Z.

Cell divisions producing two daughter cells that adopt distinct fates are defined as asymmetric. In all organisms, ranging from bacteria to mammals, in which development has been studied extensively, asymmetric cell divisions generate cell diversity. Asymmetric cell divisions can be achieved by either intrinsic or extrinsic mechanisms (Fig. 1). Intrinsic mechanisms involve the preferential segregation of cell fate determinants to one of two daughter cells during mitosis. Asymmetrically segregated factors that bind cell fate determinants and orient the mitotic spindle may also be necessary to ensure the faithful segregation of determinants into only one daughter cell. Extrinsic mechanisms involve cell–cell communication. In metazoans, a dividing’s cell’s social contex provides a wealth of positional information and opportunity for cell–cell interactions. Interactions between daughter cells or between a daughter cell and other nearby cells could specify daughter cell fate. Interaction between a progenitor cell and its environment can also influence cell polarity by directing spindle orientation and the asymmetric distribution of developmental potential to daughter cells. Recent studies have indicated that a combination of intrinsic and extrinsic mechanisms specify distinct daughter cell fates during asymmetric cell divisions. We focus on asymmetric cell divisions that occur during the development of the Caenorhabditis elegans and Drosophila nervous systems. Although we touch upon asymmetric cell divisions in the early C. elegans embryo, more extensive reviews on this subject are available (Guo and Kemphues 1996a; Han 1997; Kemphues and Strome 1997; Bowerman 1998).